ARN can't just be "moved"- they are either a part of the mesh or are a separate mesh entirely because the ARN is a set of textured planes. Some models have the ARN as part of the main mesh so the original source files would be needed (like Shane said), but the ones that have the ARN as a separate mesh can have a new ARN mesh made without the original sources. You would still need gMax/Blender/3ds Max to make the new ARN mesh. It would be a matter of trial and error when you make the new number mesh to get them to line up with a model that is not already in the game, so it would be a fairly time consuming process.
Alpha channel ARNs only work on the "default" mesh in a config from what I understand. If what I understand about them is correct, I assume it would be possible to rename the mesh from default to something else (like "locomotive") and have the ARN only mesh named "default"- I would assume that the game would use the alpha number textures on the ARN only "default" mesh and would ignore the textures on the "locomotive" mesh, but I've never tried it.
Finally, if you disable the ARN entirely, you can make single-numbered repaints......
